1. Field of the Invention
The present invention relates to a method and apparatus for forging, from a metal pipe, a rack bar, which is used for a steering apparatus in an automobile et al.
2. Description of Related Art
A rack bar as a part for a steering apparatus of an automobile is conventionally produced from a solid rod member by a machining such as a broaching. However, such a conventional rack bar is defective in an increased weight due to its solid structure. Furthermore, a rack bar of varied gear ratio (VGR) cannot be obtained by the broaching since a machining of toothed portions of varied width is impossible by the broaching.
In view of the above, in order to reduce a weight as well as to obtain a varied gear construction, a formation of a hollow rack bar from a blank pipe produced from a forging process has recent been proposed. See Japanese Un-examined Patent Publication (Kokai) No. 3-5892, Japanese Un-examined Patent Publication (Kokai) No. 5-169181 or Japanese Un-examined Patent Publication (Kokai) No. 6-246379 or Japanese Un-examined Patent Publication (Kokai) No. 2001-300677.
When a forging of a hollow rack bar is done, a blank pipe is held by a toothed die. Then, a mandrel is, under a pressure, inserted to the blank pipe held by the die, so that a radial expansion of the metal (plastic flow of metal material) is generated toward toothed portions, thereby forming a rack bar. Thus, a desired control of the cross-sectional area is essentially needed in order to obtain a desired precision of the rack bar. However, blank pipes supplied from manufacturers are, as far as their values of a inner and outer diameters are concerned, merely controlled in a range of tolerance under a government regulation, such as a Japanese Industrial Standard (JIS). Thus, a precision of these blank pipes as for the forging of rack bars are insufficient, resulting in a variation in the flow of metal as well as in an increased forming force during the execution of the forging process. Thus, a production efficiency is reduced, on one hand and, on the other hand, a service life of tools (dies) is reduced due to the excessive load occurred therein during the execution of the working.
The prior art method for producing a rack bar is defective in view of difficulties in a subsequent hardening process. Namely, in a usual hardening process, a rack bar is held by a die only at an outer diameter side. In this case, no restriction of escape of thermal distortion toward the inner diameter side is possible. Furthermore, an insertion of a core is not effective for obtaining a desired precision because of a reduced precision of an inner diameter of a blank pipe within a tolerance. As a result, a specially designed technique is required for executing the hardening of the hollow rack bar, which causes the hardening process to be time consuming, on one hand and, on the other hand, causes the production cost to be increased. Furthermore, in a rack forging process, a metal flow is likely generated in the direction where a moving distance is short, i.e., the flow of metal is easy. As a result, when a mandrel of a simplified uniform cross-sectional type is used in a rack forging process, a locally excessive or locally shortened flow of the metal to a toothed die is likely generated, resulting in reduced performances. In view of this, an improvement has been proposed, wherein a plurality of mandrels of non-uniform shapes are provided for obtaining non-uniformed flows of metal. However, this improved process is defective due to its increased cost, on one hand and, on the other hand, its inability of possibility in the adjustment of metal flow in the axial direction.